Visual Perception, Data Visualization, and Science

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Imagine someone explaining a complex topic, like how to improve the fuel efficiency of a boat. But shortly after starting the explanation, they go off on a series of tangents about pretty boats they’ve seen, big boats, rubber ducks, submarines, and other transportation vehicles such as the new 787 by Boeing et al. Then they return to the explanation of boat efficiency without ever referencing why they brought up those strange tangents.

Tangents are confusing, and they hurt clarity. The related work section is often just a string of unrelated tangents, which is a waste of the reader’s time.

Now let me make something clear: I am not necessarily saying that papers should cite fewer sources. Instead, each citation should serve an obvious, specific purpose. And if that purpose is so tangential to the structure of your argument that you need to put it in what amounts to a citation dumping ground, then it isn’t needed.

What’s the purpose of a citation?

This year, 40% of InfoVis papers included an empirical evaluation. I made a list in my last post.

There were also a couple papers worth noting that described methods for evaluating visualizations. These papers can help bootstrap future evaluations, leading to a better understanding of when and why vis techniques are effective.

Learning Perceptual Kernels for Visualization Design – Çağatay Demiralp, Michael Bernstein, Jeffrey Heer pdf
A collection of methods are described to find the relative discriminability of feature values (e.g. colors or shapes). It also looks at finding the descriminability of combinations of visual features (e.g. colors and shapes). The paper validates its approach by determining the discriminability of size and showing which of their measures closely match the established Steven’s power law for size.

A Principled Way of Assessing Visualization Literacy – Jeremy Boy, Ronald Rensink, Enrico Bertini, Jean-Daniel Fekete pdf
This paper describes how to use Item Response Theory – a technique common in psychometrics and education literature – to assess a person’s “literacy” or skill with visualizations. I would have liked to have seen the approach validated (or at least compared) with some external factor like the person’s experience with visualization. Understandably, that can be tough to measure, but this method certainly shows promise for explaining individual differences in user performance.

I did not discriminate beyond those two criteria. However, I am using a gold star ★ to highlight one property that only a few papers have: a generalizable explanation for why the results occurred. You can read more about explanatory hypotheses here.

It’s always amazing how many basic visualization questions are yet to be answered. Robert Kosara raised one yesterday: What is the most effective way to show large scale differences?

Rather than using a bar chart to represent values, he made a demo that sequentially shows dots to demonstrate how many more times a CEO makes than a worker. His solution looked compelling, but I realized that I don’t know of any literature in vis that has empirically tackled this problem. A goal as simple as visualizing a pair of values of very different scale has few (if any) guidelines.

Furthermore, although there have been a few papers on animation in charts (e.g. [2, 4]), the basic approach of using animation to represent a single value still has many unanswered questions.

Robert’s demo used both numerocity and duration of the animation to visualize each value. I forked his code to make a demo of some alternative animation styles (options at the bottom), but I don’t know of any literature that hints if or why one would be better than another:This is an interactive feature that does not work in your feed reader. Please click on the image below to go the web version to see it.Continue reading →

I have never seen this practice in any other field, and I was curious as to the origin.

Half Hypotheses

Although these statements are referred to as ‘hypotheses’, they’re not… at least, not completely. They are predictions. The distinction is subtle but important. Here’s the scientific definition of hypothesis according to The National Academy of Sciences:

A tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation…

The key word here is explanation. A hypothesis is not simply a guess about the result of an experiment. It is a proposed explanation that can predict the outcome of an experiment. A hypothesis has two components: (1) an explanation and (2) a prediction. A prediction simply isn’t useful on its own. If I flip a coin and correctly guess “heads”, it doesn’t tell me anything other than that I made a lucky guess. A hypothesis would be: the coin is unevenly weighted, so it is far more likely to land heads-up. It has an explanation (uneven weighting) that allows for a prediction (frequently landing heads-up).

The Origin of H1, H2, H3…

Besides the unusual use of the term “hypothesis”, where does the numbering style come from? It appears in many IEEE InfoVis and ACM CHI papers going back to at least 1996 (maybe earlier?). However, I’ve never seen it in psychology or social science journals. The best candidate I can think of for the origin of this numbering is a misunderstanding of null hypothesis testing, which can be best explained with an example. Here is a null hypothesis with two alternative hypotheses:

H0: Objects do not affect each other’s motion (null hypothesis)

H1: Objects attract each other, so a ball should fall towards the Earth

H2: Objects repel each other, so a ball should fly away from the Earth

Notice that the hypotheses are mutually exclusive, meaning only one can be true. In contrast, Vis/CHI-style hypotheses are each independent, and all or none of them can be true. I’m not sure how one came to be transformed into the other, but it’s my best guess for the origins.

Unclear

On top of my concerns about diction or utility, referring to statements by number hurts clarity. Repeatedly scrolling back and forth trying to remember “which one was H3 again?” makes reading frustrating and unnecessarily effortful. It’s a bad practice to label variables in code as var1 and var2. Why should it be better to refer to written concepts numerically? Let’s put an end to these numbered half-hypotheses in Vis and CHI.

Do you agree with this perspective and proposed origin? Can you find an example of this H numbering from before 1996? Or in another field?

When reading a paper (vis or otherwise), I tend to read the title and abstract and then jump straight to the methods and results. Besides the claim of utility for a technique or application, I want to understand how the paper supports its claim of improving users’ understanding of the data. So I put together this guide to the papers that ran experiments comparatively measuring user performance.

Less than a quarter

Only 9 out of 38 InfoVis papers (24%) this year comparatively measured user performance. While that number has improved and doesn’t need to be 100%, less than a quarter just seems low.

Possible reasons why more papers don’t evaluate user performance

Limited understanding of experiment design and statistical analysis. How many people doing vis research are familiar with different experiment designs like method of adjustment or forced-choice? How many have run a t-test or a regression?

Evaluation takes time. A paper that doesn’t evaluate user performance can easily scoop a similar paper with a thorough evaluation.

Evaluation takes space. Can a novel technique and an evaluation be effectively presented within 10 pages? Making better use of supplemental material may solve this problem.

Risk of a null result. It’s hard – if possible at all – to truly “fail” in a technique or application submission. But experiments may reveal no statistically significant benefit.

The belief that the benefit of a vis is obvious. We generally have poor awareness of our own attentional limitations, so it’s actually not always clear what about a visualization doesn’t work. Besides being poor at assessing our abilities, it’s also important to know for which tasks a novel visualization is better than traditional methods (e.g. excel and sql queries) vs. when the traditional methods are better.

A poisoned well. If a technique or application has already been published without evaluation, reviewers would scoff at an evaluation that merely confirms what was already assumed. So an evaluation of past work would only be publishable if it contradicts the unevaluated assumptions. It’s risky to put the time into a study if positive results may not be publishable.

I’m curious to hear other people’s thoughts on the issue. Why don’t more papers have user performance evaluations? Should they?

Neil deGrass Tyson recently noted that the 2008 bank bailout was larger than the total 50 history of NASA’s budget. Inspired by that comparison, I decided to look at general science spending relative to the defense budget. How do we prioritize our tax dollars?

This information quest also gave me an opportunity to try using Tableau to visualize the results.

With science spending in green and military spending in red, the difference is enormous. In fact annual military spending is greater than the total cost of NASA’s entire history (adjusted for inflation).

The criticism is largely centered on which colors we used, namely their luminance and contrast. The criticism is based on a misunderstanding or misreading of our paper.

We have two responses:

Target and distractor colors were selected randomly for each trial and fully counterbalanced; every target color was also used as a distractor. Color and/or luminance pop-out, and discriminability differences between targets and distractors do not explain the results. Rather, grouping modulates search efficiency: Here is a demo.

Color and luminance contrast explanations do not explain our results for motion. Here is a demo.

I wrote a new demo for some recent research on the perception of biological motion. We found that the human visual system can very effectively perceive and encode a group of moving figures without having to serially inspect each one.

On the implementation side, using VBOs instead of packing the points into textures resulted in massively redundant data (some values need two copies for each frame of motion). VTF would have significantly simplified the implementation. Luckily, the Angle Project has it implemented, and the canary build of Chromium/Chrome (version 13) has integrated the changes; Firefox should have it eventually as well. I give it 3-6 months before public release versions have it as well.

Chrome’s and Firefox 4’s default vertex shader compiler has trouble with texture sampling in the vertex shader, so the demo skips that feature for those browsers. As Al mentioned in the comments, the plan to add the capability to the WebGL engine is in place.

After multiple people asked, I decided to give WebGL a try. I’m impressed but also annoyed.Trying some features outCheck out my modified WebGL moon demo. Some credits are in the source.Overall thoughts on WebGL
Pros:

The graphics performance is an order of magnitude above any other web technology

Again, it’s really fast!

It stays fairly true to OpenGL (which is good if you’re familiar with OpenGL)

Cons:

The graphics performance is noticeably slower than a desktop app. And forget about using your CPU for anything else.

It says fairly true to OpenGL (is that antiquated, procedural, state-machine-based API the best that anyone can do?)

No released (non-beta) browser can run it by default.

It is OpenGL ES, rather than full OpenGL. Radom functions are just not implemented, but no documentation mentions what’s missing. In some cases whole features just don’t work (e.g. geometry shaders).

The crippled GLSL doesn’t have most built-in shader variables like texture coordinates and gl_normal, so you need to make your own “varying” pseudonyms.

HTML, javascript, and GLSL… ALL IN ONE FILE! Readability is lost.

Overall compatibility and tools (text completion and debugging those files) are going to be the primary determining factors in WebGL’s success. It’s early, so we’ll have to see what happens.